JPS632445A - Four-phase psk demodulator - Google Patents

Abstract

PURPOSE:To automatically compensate the phase shift of a reproduced carrier signal, by superposing the amplitude-difference voltage between a pair of orthogonal phase demodulating signals corresponding to the phase shift of a reproduced reference arrier signal upon a phase controlling voltage. CONSTITUTION:An orthogonal phase demodulating section 30 forms a pair of orthogonal phase demodulating signals from a four-phase PSK reproduced reference carrier signal (g). The pair of orthogonal phase demodulating signals from LPFs 14 and 15 before limiters 16 and 17 are sample-held by sample holding circuits 101 and 102 through a gate 103, to which the orthogonal phase demodulating signals are supplied from the limiters 16 and 17, and a voltage corresponding to the amplitude value difference of the sample held values associated with the phase shift of the signal (g) is outputted from a differential amplifier 104. The voltage is superposed upon the phase controlling voltage of the voltage controlling phase shifter 106 of a phase circuit forming the signal (g) and phase shifts of reproduced carrier signals are automatically compensated without making any manual correction, etc., to be carried out at every time.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a synchronous demodulator that demodulates a four-phase PSK modulated signal in a microwave frequency band in the fields of communication, broadcasting, and the like.

[Conventional technology]

In a signal transmission system for satellite broadcasting in the microwave frequency band, audio and video signals are time-division multiplexed and transmitted. A four-phase PSK modulation method, which is excellent in terms of occupied bandwidth and transmission efficiency, is used for audio signal transmission. For demodulation of a 4-phase PSK modulated signal, a synchronous demodulation method is mainly used in which a carrier wave signal is regenerated from a manually generated 4-phase PSK modulated signal, and the input modulated signal is demodulated using this regenerated carrier wave signal.

The method for reproducing this carrier signal is that when the frequency of the input transmission signal is high (for example, 140
Mllz), the inverse modulation method is advantageous.

Hereinafter, a four-phase PSK'4sL adjustment device using the inverse modulation method will be explained with reference to FIG. This demodulation device is roughly divided into a quadrature phase demodulation section 30, a remodulation circuit 41, and a carrier regeneration section 40 including a phase circuit 42. The input 4-phase PSK modulated signal a is demodulated in the quadrature phase demodulation section 30 using the reference carrier signal g regenerated by the carrier regeneration section 40. The modulation signal a is branched into two, and multipliers 11 and 12 divide the modulation signal into a reference carrier signal g and a reference carrier signal g.
are respectively multiplied by a signal shifted by 90'' by a 1π/2 phase shifter 13, unnecessary components other than the demodulated signal are removed by a low-pass filter 14.15, and further the demodulated data b, b and c are a pair of digital signals, which are output as the output of the demodulator and also input to the remodulation circuit 41 of the carrier wave regeneration unit 40. In the remodulation circuit 41, the input modulation signal a A signal d delayed by the delay circuit 18 by the delay caused by the orthogonal phase demodulation section 30, a signal e having a phase difference of 90'' from the double signal, and the multiplier 1, respectively.
9. Multiply by 20 and synthesize by adder 22, carrier wave signal f
Play. Since this signal f contains other frequency components,
In the next phase circuit 42, the signal is passed through a band pass filter 23 and then set to a constant amplitude by a limiter 24. This - constant amplitude carrier wave signal is subjected to phase adjustment by a manual phase shifter 25,
It is transmitted to the quadrature phase demodulation section 209 as a reproduction reference carrier signal g having a constant phase with respect to the input modulation signal a.

[Problem that the invention seeks to solve]

In microwave communication, the four-phase PSK modulated signal input to the demodulator is an IF multiplied signal that has undergone several stages of frequency conversion, and frequency fluctuations are unavoidable due to frequency fluctuations of the local oscillator. As a result, the phase in the band-pass filter 23 of the carrier wave reproducing section described above varies, so that even if the phase is initially adjusted using the manual phase shifter 25 as equipment adjustment, the phase of the reproduced reference carrier signal becomes unstable.

As a countermeasure to this problem, in Japanese Patent Application No. 149336/1984 (of which the present inventor is one of the inventors), the output of the remodulation circuit is converted to a lower frequency, and the low frequency signal is always at the center frequency of the bandpass filter. This paper presents an AFC circuit that converts the frequency back to the original frequency.

The above method provides precise phase tracking and is an excellent countermeasure, but it has the problem of increasing the circuit scale. In view of the above circumstances, it is an object of the present invention to provide a demodulator that compensates for phase fluctuations of a reproduced carrier signal to a practically acceptable level in a relatively simple method when the frequency fluctuations of a modulated signal are small. There is a particular thing.

[Means for solving problems]

As described above, the 4-phase PSK demodulator to which the present invention is applied includes a quadrature phase demodulator that demodulates a 4-phase PSK modulated signal, which is a human input signal, using a reproduced reference carrier signal, and a demodulated received digital signal. This is an inverse modulation type four-phase PSK demodulator comprising a carrier wave reproducing section that re-modulates the input signal and generates the reproduced reference carrier signal.

In the present invention, the carrier wave regeneration section is provided with a phase shift circuit that shifts the phase of the carrier wave signal regenerated by the remodulation circuit using a control voltage to make it a reproduction reference carrier signal, and the quadrature phase demodulation section has two branches. a pair of sample-and-hold circuits that sample the outputs of each low-pass filter in the path;

A gate circuit for generating a sampling signal of the sampling and holding circuit for a certain demodulated received digital signal, and means for superimposing a difference signal between the outputs of the sampling and holding circuit on the control voltage of the phase shift circuit of the carrier wave regeneration section. is provided to automatically adjust the phase of the reproduced reference carrier signal in response to frequency fluctuations of the input signal.

[Effect]

The 4-phase PSK modulated signal S (t) is S (t) = A, cos (Wct + Φ) (1)
= a, 1cos WCL+ b 1lstn Wc
t (21), which has two orthogonal components, and the coefficient of each component corresponding to the digital signal is determined by the phase Φ.In 4-phase PSK, the phase is divided into four, and each phase has (an
, bn). Therefore, the phase transition diagram of S (t) is as shown in FIG. Here, the words in parentheses are (an, b
n) and la, l for simplicity.

Let lb and l be 1.

Next, the correspondence between Φ and (an, bn) will be shown.

a, b, l S (t) 1 1 A+cos(Wct-π/4)-11
A +cos(Wct-3π/4)-1-L A
+cos(Wct -5π/4) (3)1
In the - I A , cos (Wet -7π/4) inverse modulation method, as shown in FIG. 3, orthogonal phase demodulation is performed using a reproduced reference carrier signal. This circuit is the same as that shown in FIG. 5, and the same reference numerals are used. A, B, . . . , F indicate each point shown in the figure. The signal at each point is as follows.

A: Alcos (WCt+Φ) °Azcos
WctB: A+cos(Wct+Φ) ・A゛zsinWctC: A, cosΦ D: -A, sinΦ A is determined by A+, Az, and the characteristics of the low-pass filter 14゜15. Signals at points C and D are limiter 16.1
7 is output as 1 or -1, so the value at each point for each phase is as follows.

Φ 0 point D point E point F point -π/4
A4 A4 1 1-3/4π −
A4A4-11 -5/4π -A a A 4 1 1-
7/4π A a A a 1 1 where A, = A3/~9.

Now, the phase of the reproduced reference carrier signal is displaced by θ, and A,
If it becomes cos(Wct+θ), 0 points.

The value of point D is A, cos(Φ-〇), -A, sin
(Φ−θ).

θ is the minute phase displacement to
/ 4) When demodulating the modulated wave signal, the signal value at each point is 0 point D point E point F point θ = +△ A a < Aa > 1 1
θ=-△ A a > A 4 < 1
The relationship becomes 1. Note that A4> is A, smaller than A4, and A4< is A
Indicates that it is greater than 4. The magnitude relationship between the 0 point and the D point can be seen immediately in FIG. Changes in the signal values at point 0 and point D are in opposite directions, and the difference between them is proportional to θ as long as the displacement is small, and its positive and negative values are related to the positive and negative values of θ. The signal values at points E and F do not change as long as θ is small.

In the present invention, the difference in the amplitude value between the 0 point and the D point with respect to the displacement of θ is detected, and the voltage difference is used to superimpose it on the control voltage of the voltage-controlled phase shifter in the phase circuit. By advancing or delaying the phase of the carrier wave signal, phase control is automatically performed and the signal is used as a reproduction reference phase signal.

In order to detect the difference in amplitude values between the 0 point and the D point, first the 0 point and the D point are detected.
The signal at point D is sub-ringed, and the sampling signal for this purpose is generated by the gate circuit when both points E and F become "1".

By the way, the fluctuation of the amplitude value with respect to θ = ±△ shown above (that is, the amplitude fluctuates in the opposite direction between the 0 point and the D point)
is the variation of the point at π/4 of both sin and cosine signals or an odd multiple thereof, and Φ=-π/4+-3/4π, - that is, it can be said for any phase of the 4-phase PSK modulated signal. It is.

Also, the amplitude variation is maximum at these phases, and therefore the amplitude difference is also maximum. - In general, data is scrambled during data transmission, so the probability of appearance of (1, 1), -. May be sampled.

〔Example〕

Hereinafter, one embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows a circuit block diagram of the embodiment, and the configuration of the quadrature phase demodulation section 30 is exactly the same as that of the conventional example (FIG. 5), and the reference carrier signal g regenerated by the carrier regeneration section 4°
The input 4-phase PSK modulated signal a is multiplied by the input 4-phase PSK modulated signal a and a signal orthogonal thereto in a multiplier 11.12, respectively, and demodulated data b and c are obtained by a low-pass filter 14.15 and a limiter 16.17. Demodulated data b.

At C, the input modulation signal a is further delayed by the delay circuit 18, and the modulation signal d is modulated by the re-modulation circuit 41, thereby reproducing the carrier wave signal f. This remodulation circuit 41 is exactly the same as the circuit shown in FIG.

The signal f is filtered by a bandpass filter 23 to remove signals other than the carrier frequency, and a limiter 24 converts the signal f to a constant amplitude signal f.
The configuration of the voltage-controlled phase shifter 106 uses a variable capacitance diode 106 (1) in this example, and the amount of phase shift is determined by changing the applied voltage. First, as an initial adjustment, switch 10
5 is left open and the variable voltage 107 is adjusted. The phase adjustment is performed so that the output signal of the voltage-controlled phase shifter 106 becomes the reproduction reference carrier signal g with respect to the 4-phase PSK modulated signal that is input to the demodulator.

Then, by closing switch 105, adder 10
A differential voltage k for automatic phase control is added to 8. This differential voltage includes two branches (cos component,
Sample and hold circuits 101 and 10 output the outputs of the low-pass filters 14 and 15, respectively related to the sine component.
2, and lead its output to a differential amplifier 104,
This is the output obtained. As described in the section of the operation, there is a voltage difference between the 0 point and the D point in FIG. 3, which is caused by the phase shift of the reproduced reference carrier signal g. The sampler signals of the sampling and hold circuits 101 and 102 are generated when demodulated data b and c both become 1 (Φ=-
(corresponding to π/4) in the gate circuit 103.

The differential voltage is added via an adder 108 to the bias voltage (voltage accumulated by initially adjusting the variable voltage 107) applied to the voltage-controlled phase shifter 106. Whether the voltage to be added is added positively or negatively depends on which demodulated data is sampled, and the voltage side 'qB
Although it is related to the circuit configuration of the phase shifter 106, by forming a negative feedback loop in response to a phase change, a phase shift can be automatically corrected.

〔Effect of the invention〕

As explained in detail above, in this demodulator, even if there is a frequency fluctuation of the input 4-phase PSK demodulated signal and the phase of the carrier signal to be reproduced shifts inside the demodulator, the phase is automatically changed. The deviation is corrected using a voltage controlled phase shifter. Therefore, correct demodulation is always possible. As long as the phase shift is small, the correction accuracy is high and correction can be performed linearly.

The circuit configuration is extremely simple and can be realized by adding a small number of circuits to a conventional inverse modulation type demodulator.

[Brief explanation of drawings]

FIG. 1 is a circuit block diagram of an embodiment of the present invention, FIGS. 2 to 4 are diagrams for explaining the present invention, and FIG. 5 is a circuit block diagram of a conventional example. 11.12, 19.20--multiplier, 13.21--
--π/2 phase shifter, 14, L5-low pass filter, 16.17...limiter, 18-delay circuit, 22
- adder, 23 - band pass filter, 24 - limiter, 25 - manual phase shifter, 30 - quadrature phase demodulator, 40 - carrier regeneration unit, 41 - remodulation circuit, 42 -
Phase circuit, 101.102・-Sampling hold circuit, 103
-Gate circuit, 104--Differential amplifier, 105-...
switch, 106-voltage controlled phase shifter, 10'l-m
-Variable control voltage, 106(1)--Variable capacitance diode, 108--Adder.

Claims (1)

[Scope of Claims] A quadrature phase demodulation section that demodulates a four-phase PSK modulated signal that is an input signal using a reproduced reference carrier signal, and re-modulates the input signal using the demodulated received digital signal,
In an inverse modulation type four-phase PSK demodulator comprising a carrier wave regeneration section that generates the reproduction reference carrier signal, the carrier wave regeneration section is configured to cause the carrier wave signal regenerated by the remodulation circuit to shift the phase of the carrier wave signal using a control voltage. In addition to providing a phase shift circuit to use as a reference carrier signal, a pair of sampling and hold circuits to sample each output of the low-pass filter in the two branches of the quadrature phase demodulation section, and further comprising a gate circuit for generating a sampling signal of the sampling and holding circuit, and means for superimposing a difference signal of the output of the sampling and holding circuit on a control voltage of a phase shift circuit of the carrier wave reproducing section, and controlling the frequency fluctuation of the input signal. A four-phase PSK demodulator is characterized in that it automatically adjusts the phase of a reproduced reference carrier signal.